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Curtis B.
Curtis B., Technician
Category: Small Engine
Satisfied Customers: 13581
Experience:  have worked on and around most engine models for 35 yrs./Polaris ATV expert
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My JD425 w/FD620 engine is giving me problems. It starts

Customer Question

my JD425 w/FD620 engine is giving me problems. It starts fine and runs great for about 15 minutes. Then stops dead like it was turned off. While trying to restart it, it will backfire once or twice, the engine wants to run backward for a few seconds until it dies (verified with pushing the forward hydrostatic pedal and by watching the fan belt) There will be a point where white smoke comes off the left side of the engine. cont
JA: Do you have fuel in the tank? If so, is the shut-off valve open and the fuel clean?
Customer: after many attempts at restarting, it will run fine at less than half throttle above that, it surges for a several minutes. when it clears up, it will run great for the rest of the day
JA: Do you plan on doing the work yourself?
Customer: yes, non ethanol fuel in the tank, recently changed the fuel filter, had the cam shaft changed along with coils and plugs. I've gone through the carburetor cleaning the screw out jets this past weekend
JA: Anything else we should know to help you best?
Customer: I try to do as much as I can myself both to same money and so as not to loose operating time with it (gotta keep the grass mowed)
Submitted: 9 months ago.
Category: Small Engine
Customer: replied 9 months ago.
Oh, and to answer the previous question I had about air in the transmission causing it to foam, turns out that it had bad seals on the hydraulic pump allowing it to suck air directly into the pump. That operation turned into rebuilding the transmission replacing seals and bearings thru out.
Expert:  Bill McCusker replied 9 months ago.

Hello My name is***** will do what i can to help you today.

You have alot going on here so let me ask you a few questions.

As far as the running does the unit have a fuel pump? and if so is it working?

i know you have checked the valves and cam so im guess the timming is correct? is the fly wheel key good?

Customer: replied 9 months ago.
it has a fuel pump. It can be heard running. Over the weekend, I pulled the fuel hose from the carburetor to verify fuel is flowing. Not sure how much pressure it should have though. When I had the camshaft replaced, I asked that the guy check the flywheel key, he said it looked good. I want to look for myself, just need to find the time.
Expert:  Bill McCusker replied 9 months ago.

going back to the idea of the engine going backwards it sounds like an out of timing problem but not so much so that it has it has run.. My other thought is that there is also a compression problem. but as you said you adjusted the valves. Have you checked the compression with a compression test?

Customer: replied 9 months ago.
I was thinking a timing issue which is why I want to double check the flywheel key. I had not considered compression, that is intriguing, can you suggest a desired pressure I should look for or just that both cylinders are similar. I will try checking that this weekend.
Expert:  Bill McCusker replied 9 months ago.

I will have to double check the service books at work. But i think they are about 90-120psi.. Let me double check and i will message you tomorrow..

Customer: replied 9 months ago.
Thank you for any additional information you can provide
Expert:  Bill McCusker replied 9 months ago.

according to the service manual. 170psi min for compression.

Expert:  Bill McCusker replied 9 months ago.

did this answer your problem?

Customer: replied 9 months ago.
Unfortunately, I didn't get much accomplished over the weekend. Couldn't find my compression tester and rounded the head off a bolt trying to get to the flywheel key. Stopped at the local JD dealer for a new bolt and a flywheel key, they suggested the problem could be caused by a bad time delay module. Still working on getting that bolt out.
Expert:  Bill McCusker replied 9 months ago.

ok i thought i was a timing problem.. let me know..

Customer: replied 9 months ago.
Hi Bill,
I think I might have this one licked. Got that stubborn bolt out. Before I tear into it further in order to get the flywheel off, I thought I'd try the delay module. A sheared flywheel key just didn't feel like the right solution either. That seems to have made a difference, ran for over half an hour without problems (longer than it has on initial start in weeks). And it seems to run stronger than it has in quite a while.
Customer: replied 9 months ago.
I mowed grass until dark. I'll know more this weekend when I have more time on the machine. And, of course, only time will tell if it's actually fixed.
Expert:  Bill McCusker replied 9 months ago.